Method and apparatus for fractionating air and power production



July 10, 1956 F. JAKOB 2,753,698

METHOD AND APPARATUS FOR FRACTIONATING AIR AND POWER PRODUCTION Filed March 3, 1953 INVENTOR:

FRI T Z JAAOB B) K. W

ATTOP/VEK United States Patent l METHOD AND APPARATUS FOR FRACTIONAT- ING AIR AND POWER PRODUCTION Fritz Jakob, Munich-Solln, Germany, assignor to Gesellschaft fuer Lindes Eisrnaschinen Aktiengesellschaft, Hollriegelskreuth, near Munich, Germany, a corporation of Germany Application March 3, 1953, Serial No. 340,033 7 Claims priority, application Germany March 5, 1952 6 Claims. (Cl. 62-123) The use of turbines for producing the power required to produce the cold needed in air fractionating plants is limited because the volume of the available turbine operating medium is insuificient for the efficient operation of a turbine. The present invention relates to an air fractionating method and apparatus which provides enough volume of operating medium for the turbine even if relatively small amounts of air are fractionated.

The method according to the invention includes the step of preliminarily fractionating the total amount of air in the pressure column of a double rectifier, expanding the impure oxygen, which is taken from the bottom of the pressure column, to an intermediate pressure which is so high that the impure oxygen can be evaporated by heating with nitrogen in the gas state which condenses at the head of the pressure column. The oxygen vapor is heated in the conventional manner by heat exchange with a warmer gas and is expanded in a turbine and conducted into the upper column of the fractionating apparatus.

It is possible to evaporate the impure liquid oxygen pro duced in the pressure column of a conventional fractionating apparatus at a pressure between 2.6 and 2.8 atm. absolute by heat exchange with condensing nitrogen produced in the pressure column. A temperature difference of at least 2.5 C. is available for the heat transfer from the condensing nitrogen to the evaporating oxygen.

, The difference between the new method and the conventional method resides in the fact that in the new method no or only little impure oxygen liquid is expanded so that the rectification in the bottom part of the columns is somewhat less favorable. However, the efficiency of scrubbing with liquid nitrogen in the upper part of the column is considerably increased. The less favorable rectification in the lower part of the column is more than compensated by the improved rectification conditions in the upper part of the column. The new method has the same fractionating effect as conventional methods in spite of the considerable change of the rectification conditions.

The method according to the invention is much more economic than the conventional method. The cold produced by the expansion of one normal cubic meter air circulated through the apparatus from 2.7 atm. absolute to 1.3 atm. absolute amounts to 1.88 kg.-cal. at a turbine efliciency of 70%, provided that 48.4% of the air is expanded in the turbine in the form of impure oxygen.

The conventional method in which a considerably smaller part of the air, 23.6% of the total amount, is expanded from 5.5 atm. absolute to 1.3 atm. absolute yields a cold production by the turbine of only 1.82 kg.-cal. per normal cubic meter of the total amount of air. With the new method the more favorable value of the specific cold output, when expanding impure oxygen, is closer to the lower limit, because, in the new method, the turbine efiiciency is greater due to the smaller pressure drop. The new method produces an improved result without decrease of the fractionating effect, because the total air is prefractionated, in contradistinction to the conventional air supply, the amount of the scrubbing nitrogen being 2,753,698 Patented July 10, 1956 increased by the amount available from the air blown-in in the conventional method. It is not always necessary or possible to evaporate all impure oxygen and expand it for power production; under certain circumstances, it is sufiicient that the major part be expanded, the smaller part being directly introduced into the upper column, bypassing the evaporator and turbine.

The novel features which I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself however and additional objects and advantages thereof will best be understood from the following description of an embodiment thereof when read in connection with the accompanying drawing which shows a diagrammatic lay-out of a plant according to the invention.

Referring more particularly to the drawing, the air to be fractioned is brought to a pressure slightly higher than the liquefaction pressure, for example to a pressure of 5.7 atm. absolute, by means of a rotary compressor 35, whereupon it is transferred, by means of a conduit system 18, to the high temperature ends of two regenerator pairs 19 and 20. By means of a conduit system 21, cooled air is taken off the low temperature ends of the regenerators, carried to a heat exchanger 22 and from there to a high pressure column 1 of a fractionating apparatus. The regenerators can be of the usual type, or be constructed like, for instance, the regenerators disclosed in U. S. Patent No. 1,970,299. A small portion of the air to be fractioned is branched off at 23 from the main regenerators 19 and transferred to reversible adsorbers 24, where it is purified in the same manner as the balance of the air passing through the entire length of the regenerators. This branching-off of part of the air stream is a practical means of retaining the heat-exchange capacity of the main regenerators at the desired level. After passing from the adsorbers 24 through the conduit 25 and the heat exchanger 9, this partial air volume is reunited with the main stream and, together with the latter, enters, at a pressure of 5.5 atm. absolute, through the inlet 3 into the high pressure column 1 of the fractionating apparatus.

The high pressure column 1 represents the lower part of a double rectifier 1, 2 where the cooled and compressed air is preliminarily fractionated to form'impure oxygen and pure nitrogen. The latter is condensed in the head of the column by heat exchange partly with evaporating pure oxygen and partly with impure oxygen. The oxygen accumulating at the bottom of the pressure column is removed through conduit 8, expanded in a valve 7 to the evaporating pressure, completely evaporated in an evaporator 4, heated in a heat exchanger 9 by warmer air flowing through conduit 25, expanded for power production in a turbine 10 to the pressure in the upper column 2, and introduced into the latter through a conduit 11. Heating of the impure oxygen may be effected, for example, by the aforementioned air from the regenerators 19 which had been only partly cooled. Within the heat exchanger 9, this branched-off portion of air is cooled to saturation temperature and is even liquefied to a small extent while transferring its heat content to the gaseous, impure oxygen coming from evaporator 4. Another possibility of heating the oxygen to be expanded is to conduct the oxygen in tubes through a relatively cold part of the regenerator for heat exchange with air to be fractionated. It is, however, unnecessary to pass the entire impure oxygen through the heat exchanger 9 and the turbine 10. By means of a conduit 32, a fraction of the impure oxygen can be by-passed to enter, after expansion through valve 33, the upper column 2 at 34; mostly in form of liquid. The liquid nitrogen produced in the pressure column is removed through a conduit 5 and is undercooled by heat exchange in a counterflow heat exchanger 6 with pure nitrogen discharged from the upper column 2 through conduit 12. The pressure of the liquid nitrogen is reduced in a valve 13 to the pressure in the upper column and the nitrogen is introduced into the upper column through a conduit 14 to serve as a scrubbing agent. By means of the counterflow heat exchanger 6, gaseous nitro gen leaving the low pressure column at 12 is heated to the temperature of liquid nitrogen within the high pressure column. At the same time, the nitrogen within the high pressure column is undercooled. The afore-mentioned nitrogen portion from the low pressure column 2 is carried to a heat exchanger 22 where it lowers the tempera ture of the main air stream within the conduit 21 to the saturation point. A conduit 28 transfers the nitrogen from the heat exchanger 22 to the regenerators 19, to enter there, in the known manner, into a caloric exchange with the heat absorbing material contained therein. Jy means of a conduit 29, the nitrogen leaves the regenerators as one of the final products of the fractionating process.

Pure oxygen which collects at the bottom of the low pressure column 2 is taken off at 15 and, by means of a conduit 30, is taken to the regenerators 20 from which, after having acted as a coolant the same way as the nitrogen, it is removed by means of a conduit 31 as the other final product of the fractionating process.

The method and apparatus according to the invention is not limited to the use of turbines but can be performed with any kind of suitable expansion machines.

While I have shown and described a specific embodiment of my invention, it will be apparent to those skilled in the art that various changes, modifications, substitutions, additions and omissions may be made therein without departing from the spirit and scope of my invention as set forth in the appended claims.

What is claimed is:

1. A method of fractionating air in which the total air to be fractionated is prefractionated in the pressure column of a double rectifier, the so produced impure liquid oxygen is expanded to a medium pressure and is com.- pletely evaporated in the pressure column by heat exchange with condensing nitrogen, the evaporated oxygen is slightly heated, expanded against a load for power production, and scrubbed in the upper column of thedouble rectifier by the nitrogen condensed within the high pressure column and entered on top of the low pressure column.

2. A method as defined in claim 1, in which the initial pressure of the impure liquid oxygen is substantially 5.5

.4 atm. absolute, the medium pressure to which the impure liquid oxygen is expanded is between 2.6 and 2.8 atm. absolute, and in which the oxygen is expanded for power production from said medium pressure to the pressure in the upper column of the double rectifier.

3. A method according to claim 2, the pressure in the upper column being substantially 1.3 atrn. absolute.

4. An apparatus for fractionating air comprising a double rectifier having a pressure column and an upper column, an inlet in said pressure column for the total air to be fractionated, an evaporator in the upper part of said pressure column, a conduit connecting the lower part of said pressurecolumn with said evaporator for conducting impure liquid oxygen into said evaporator, an expansion means in said conduit for reducing the pressure of the impure liquid oxygen to a medium pressure, said evaporator being adapted to be heated by condensation of nitrogen produced in said pressure column, a turbine, a conduit connecting said evaporator with said turbine for supplying evaporated impure oxygen to said turbine, a conduit connecting said turbine with said upper column for conducting evaporated oxygen after expansion in said turbine into said upper column, and conduit means connected with and receiving nitrogen condensed by said evaporator and connected with and conducting the condensed nitrogen into said upper column to serve as a scrubbing liquid in said upper column.

5. An apparatus according to claim 4, comprising a cooler interposed in said conduit means and connected with said upper column for receiving pure nitrogen therefrom as a cooling agent, and a pressure reduction means in said conduit means for reducing the pressure of the liquid nitrogen produced in said pressure column to the pressure in said upper column.

6. An apparatus according to claim 4, comprising a bypass conduit connected with the lower part of said pressure column for receiving a minor portion of the impure liquid oxygent and connected with said upper column for directly conducting the minor portion of the impure liquid oxygen into said upper column.

References Cited in the file of this patent UNITED STATES PATENTS 1,620,192 Claude Mar. 8, 1927 2,040,107 Schlitt May 12, 1936 2,520,862 Swearingen Aug. 29, 1950 2,668,425 Skaperdas Feb. 9, 1954 

1. A METHOD OF FRACTIONATING AIR IN WHICH THE TOTAL AIR TO BE FRACTIONATED IS PREFRACTIONATED IN THE PRESSURE COLUMN OF A DOUBLE RECTIFIER, THE SO PRODUCED IMPURE LIQUID OXYGEN IS EXPANDED TO A MEDIUM PRESURE AND IS COMPLETELY EVAPORATED IN THE PRESSURE COLUMN BY HEAT EXCHANGE WITH CONDENSING NITROGEN, THE EVAPORATED OXYGEN IS SLIGHTLY HEATED, EXPANDED AGAINST A LOAD FOR POWER PRODUCTION, AND SCRUBBED IN THE UPPER COLUMN OF THE DOUBLE RECTIFIER BY THE NITROGEN CONDENSED WITHIN THE HIGH PRESSURE COLUMN AND ENTERED ON TOP OF THE LOW PRESSURE COLUMN. 